1. Field of the Invention
The present invention relates to an optical fiber.
2. Description of the Related Art
In an optical communication system, it is desirable to improve an optical signal-to-noise ratio (OSNR). The improvement in OSNR is particularly important for a communication using a digital coherent detection. By improving OSNR, the capacity of transmission signals can be increased, the transmission distance of the optical communication system can be increased, and the interval between repeaters can be increased. Thus, the performance of the optical communication system can be improved.
To improve OSNR, it is important to reduce the non-linearity of an optical fiber used as an optical transmission line and to reduce the attenuation in the optical transmission line. To reduce the non-linearity generated in the optical fiber, the effective area Aeff of the optical fiber may be increased and the absolute value of chromatic dispersion of the optical fiber may be increased. There is known a non-dispersion shifted optical fiber in which the absolute value of chromatic dispersion is large and the effective area Aeff is increased (for example, International Publication No. 00/062106, Japanese Published Unexamined Patent Application No. 2005-202440, and International Publication No. 2011/066063).
However, the non dispersion shifted optical fiber with the increased effective area Aeff has a large dissimilar splice loss with respect to a standard single mode fiber (SSMF, an effective area Aeff in a wavelength band of 1.55 μm being about 80 μm2) that is the ITU-T G.652 series, a dispersion shifted fiber (DSF, an effective area Aeff in a wavelength band of 1.55 μm being in a range from 50 μm2 to 80 μm2) that is the ITU-T G.653 series, and a non-zero dispersion shifted fiber (NZ-DSF) that is the ITU-T G.655 and G.656 series, these fibers which are already installed as optical transmission lines or used for optical fiber based devices in transmitters, receivers, repeaters and so on. As the result, OSNR may be decreased. A standard optical fiber of the ITU-T G.652 series has optical characteristics, such as a cable cutoff wavelength being 1260 nm or shorter, a nominal value of a mode field diameter (MFD) at a wavelength of 1310 nm being in a range from 8.6 μm to 9.5 μm, a zero-dispersion wavelength being in a range from 1300 nm to 1324 nm, and a dispersion slope at the zero-dispersion wavelength being 0.093 ps/nm2/km or smaller.
U.S. Pat. No. 6,421,489 discloses optical fibers having a large effective area Aeff and a small mode field diameter MFD in Table-I. However, these optical fibers are dispersion shifted fibers with a zero-dispersion wavelength in a range from 1508 nm to 1570 nm. Hence, the optical fibers have a small absolute value of chromatic dispersion, and the non-linear phenomenon likely occurs. Also, these optical fibers are expected to have a high bend loss (more particularly, micro-bend loss). In addition, these optical fibers may have a very long cutoff wavelength of 1857 nm or larger.
U.S. Pat. No. 6,687,441 discloses optical fibers having a large effective area Aeff and a small mode field diameter MFD in Table-I. However, these optical fibers are dispersion shifted fibers with a zero-dispersion wavelength in a range from 1472 nm to 1579 nm. Also, these optical fibers contain a void at the center. Hence, it is expected that the productivity may be not high and the attenuation may be large. In addition, it can be easily expected that the splice loss be increased because the void of these optical fiber is collapsed during fusion splicing with other optical fiber and hence the waveguide structure is changed.